Monday, January 3, 2011

Today I implemented ambiguous position support in aprs.fi. Many APRS transmitters using MIC-E or uncompressed packets can be configured to intentionally transmit less precise positions. This may seem a bit backward at first, but there are perfectly good reasons to do so. Some people might want to transmit a rough location of their car without revealing the exact parking spot where their expensive radio gear spend their night in. Some might like some aspects of APRS but wish to adjust the level of privacy by hiding their precise location.

Ambiguity is configured by setting the number of digits which will be truncated from the end of the position. Plaintext APRS positions are transmitted in degrees and decimal minutes (DD° MM.mm'), with two decimals of minutes. When ambiguity is set to 1, it'll be truncated to DD° MM.m', 2 will transmit DD° mm', 4 will transmit DD° only, resulting in a resolution of 1 degree.

Stations transmitting ambiguous positions are now shown with a purple dashed border around the callsign label. The precision of the position can be visualized by hovering the mouse pointer on top of the station symbol – a purple rectangle will pop up, showing the area of ambiguity. The station symbol is drawn at a slightly randomized position within that area. Also, the info page now says "Position ambiguous: Precision reduced at transmitter by 3 digits, position resolution approximately 18.5 km."

There is one known big bug in this code: the ambiguity is not yet stored together with each position in the position history database. Only the ambiguity of the most recently received position is stored. If you turn ambiguity on or off, the effects on aprs.fi might be slightly weird when looking up old tracks. I'll fix that later when I next change the position history table schema.

I've been told some Kenwood mobile rig owners have accidentally enabled ambiguous positions in their transmitters without realizing what the setting actually does. This change should make the effect visible in a more meaningful way. On the TM-D710, the setting lives in menu 606 (BEACON INFORMATION), POSITION AMBIGUITY.

Off-topic news: Kukka (FIFE GIC Moosegrove Orqideas Doiradas), our lovely chocolate spotted ocicat, is expecting kittens in late January. The sire is CFA CH Windhaven Salt and Pepper, an immigrant from the USA (family tree). She's gained quite some weight, as can be seen in this photo. Kittens for sale later this spring, details at the cattery site!